Clinicians anticipated that with an increase in implant number, the maximum strain value in peri-implant bone would decrease and the strain in the bone would be more widely distributed。 This was based on the assumption that when adding more implants for anchorage and support, the force borne by each implant would decrease, resulting in a decrease of strain in the bone。 Nevertheless, according to our results, the peak strain value in peri-implant bone increased with the increase in number of implants in models A, C and D under three loading conditions。 This can be explained by the increase in the supporting effect of the implants。 In the single- implant overdenture, most of the force was loaded on the mucosal area。 With increased implant numbers, more of   the

chewing force was shared by the implants while less was borne by the mucosa, resulting in the increased peak strain values in cortical bone around the implants。 This is also the reason why single- and two-implant overdentures are called ‘‘implant-retained overdentures’’, while overdentures on four implants are called ‘‘implant-supported overdentures’’。

Our study showed that under VI, which simulated the action of cutting food with the anterior teeth, the maximum stress value in the abutments in model B was three times higher than in the other three models, suggesting that possible damage to the abutments might happen more easily in two- implant overdentures than single, three and four-implant overdentures。 Kimoto et al。 also reported rotational movement around the fulcrum line between the two implants in some of their edentulous patients rehabilitated using two-implant- retained overdentures。31 However, due to the scarcity of literature concerning the effects of implant number on stress distribution in the upper structure of the overdenture, further experimental stress analysis and long-term clinical research needs to be carried out。

It can be postulated that forces, both axial and lateral, generated by an overdenture on a single implant, have the potential to be greater than those produced by a multiple implant-retained/supported overdenture, resulting in a risk of loss of osseointegration。 However, Maeda et al。 evaluated the biomechanical rationale for single-implant mandibular over- dentures using magnetic and ball attachments in an in vitro model and found that single-implant overdentures had biomechanical properties similar to two-implant overdentures in terms of lateral forces to the abutment and denture base movements under functional molar loads。32 Our study showed that stress in the abutment of model A was lower than in the other three models under three loading conditions。 Moreover, when functioning with anterior teeth, the overdenture an- chored by a single implant rotated over the implant from one side to another and randomly inclined to one side, which in our case happened to be the left side。 A similar effect happened under VM with the same model。 Thus, the left side of the whole alveolar ridge took the role of bearing the occlusal load。 The contact area between the denture and mucosa in model A was therefore much larger than in the other three models, causing less pressure on the mucosa。 In addition, under VI, the maximum equivalent strain in peri-implant cortical bone in model A was much lower than in the other three models, indicating that denture loading did not cause any apparent increase of strain in peri-implant bone and that the implant mainly took the role of retention rather than support。 Therefore, our results suggest that use of a single-implant overdenture does not lead to strain concentration in the bone around the implant and could be a feasible choice for edentulous patients。 Clinical studies also suggest that mandibular single-implant overdentures are a successful and beneficial treatment option for older edentulous adults with minimal financial outlay。6–11 Two-implant overdenture has been considered a first choice for the treatment of edentulous patients worldwide。 However, we found that under anterior loading, the denture showed more obvious rotation than it did in models C or D。 This agrees with another study showing that the application of three or four implants may create an angular instead of a straight-line  relationship  between  the  implants, preventing